Brief outline of Pennsylvania’s Geologic History

Kurt Friehauf

360-285 million years ago – Carboniferous passive margin

The Carboniferous period of geologic history is an important
one for Pennsylvania.The rocks from
this period are so well preserved and important that geologists all over
America refer to the later half of the Carboniferous period as the
Pennsylvanian period!The reason the
rocks are so well preserved in Pennsylvania lies in the geography of the area
at the time.Eastern Pennsylvania was
still pretty mountainous at the time – mountains that formed during the Taconic
and Acadian Orogenies.Western
Pennsylvania, however, was submerged beneath a shallow sea that was cut off
from the ocean by those eastern mountains.Sediments washed off of the mountains into that sea, creating a thick
pile of well-preserved sedimentary rocks.The margins of that sea were thickly-vegetated swamps.Those swamps would later play a major role
in the history of the state because those deposits of dead and decayed plant
material ultimately were turned into Pennsylvania’s famous coal deposits!Pennsylvania coal fueled the both the
steel-making mills and the cement plants during the industrial revolution.

Further north, the Catskill delta was depositing in New
York.

Far off in the distant east, though, on the other side of
the ocean, subduction of the oceanic plate was closing the gap between North
America and Africa/Eurasia.The light
at the end of that tunnel was the headlight of an on-coming train!